If you no longer wish to use BlogAssist, or want to update it to a new version, you need to quit the application. Although BlogAssist doesn't show up in the Dock, it is still a normal Mac application. It uses a small system menu on the right-hand-side of the menubar.
The last item in this menu is the Quit BlogAssist command. Choose this, and BlogAssist will quit (optionally with a confirmation).
You can then either trash the BlogAssist application from your Applications folder, or replace it with a newer version.
If you replace it, double-clicking on the new version will add the system menu back again.